The short answer
A flat rooflight does not need a pitch of its own so much as a roof that falls. A flat roof should be laid with a fall to its outlets, and the rooflight sits on an upstand that stands on that slope. The glass itself usually has a minimum pitch set by the manufacturer, often a few degrees, so that water runs off the surface instead of lying on it. The number varies by product, so always check the datasheet for the unit you are buying.
Both ideas matter: the fall of the roof, and the minimum angle of the glass. They are separate, and a rooflight can fail on either one.
The fall of the roof around it
Flat roofs are never meant to be perfectly level. Guidance for flat roof design, such as BS 6229, recommends a design fall of 1:40, with 1:80 as the lowest finished fall after allowing for deflection and tolerances. In plain terms, that is about 25mm of drop for every metre run at design stage. Anything less and water begins to pond, which shortens the life of the membrane and puts extra weight on the roof.
The roof around a rooflight is where the fall matters most. Water running down the roof meets the upstand on the high side and has to be carried round it. A rooflight set on the slope with a dam of water behind it on the upslope face is a common source of trouble. Part of the job is the layout: where the outlets are, how the roof falls to them, and whether a small tapered insulation detail, sometimes called a cricket or a saddle, sends water round the upstand.
The minimum pitch of the glass
Flat glass rooflights, including frameless types, are sold as suitable for flat roofs, and a flat roof is not flat. The glass sits above the upstand, and the manufacturer states the range of pitch it works within, from a lower limit up to some higher figure. The lower limit is there because glass at too shallow an angle holds a film of water and dirt, and stagnant water at the edges attacks sealants.
Some units meet this by being built with a slight tilt in the top face, so that the glass itself has a fall even where the roof is level. Others rely on the upstand being formed with a built-in slope. Both approaches exist, and the product literature tells you which is which.
Because published limits vary, we do not quote one figure for every product. When we survey, we measure the roof's actual fall, and the lantern or rooflight that suits it is chosen against the manufacturer's stated range.
What if the roof is dead flat?
Older flat roofs on extensions and garage conversions around Exeter, especially from the 1960s and 70s, are often close to level, or have settled over time so that parts hold water. A rooflight on a roof like that needs one of two answers.
- Re-lay the roof with tapered insulation, so that it falls properly to the outlets, with the rooflight upstand built in the same operation.
- Choose an upstand and glazing designed to give the glass its own tilt, for a roof that cannot be altered.
The first is better where the roof covering is near the end of its life, since the work is shared. The second is a compromise, and it needs checking carefully for ponding on the roof around the base.
Pitched roofs and the other end of the range
The same rooflight products have an upper limit too. Beyond a certain angle, a flat rooflight is no longer in its design range and a roof window or a pitched-roof product is the right choice. If your roof is steeper than a flat roof but not steep enough for a slate or tile, such as a low-pitched mono-pitch extension in lead or zinc, the choice of product depends on the angle. Our guide to flat rooflights or roof lanterns goes through the shapes available, and the main page on flat roof rooflights covers where each suits.
The upstand and how it sets the angle
The upstand, or kerb, is the box on which the rooflight sits, and it governs the final angle of the glass. A kerb built on a sloping roof can be formed level on top, or can follow the slope. On a roof with a 1:40 fall, a level kerb means the glass is level, and that is outside the working range of some units. A kerb with a tapered top, or a unit with an integral tilt, brings the glass back within it.
Height matters here, too. A taller upstand lifts the glass clear of surface water and makes the membrane upstand easier to dress. Our guide to flat rooflight upstand heights sets out how that is worked out, and the page on fitting rooflights on GRP, EPDM and felt shows how each covering meets the upstand.
Checking your own roof
You can get a rough idea before anyone visits. After rain, look at the flat roof from an upstairs window or a ladder at the eaves. Water still standing a day later, a tide-mark of dirt, or moss in one patch means the roof is not falling properly there. Take photos of those patches, note where the outlets or gutters are, and tell us whether the roof has been relaid since the extension was built. A spirit level and a straight board laid along the roof give a crude reading too, though a survey measures it properly. It is much easier to correct a fall while a roof is being relaid than to chase it afterwards.
What it means inside the room
The roof's fall changes what happens below, as well. If the upstand is tapered, the lining inside is cut to follow it, and the ceiling opening is built to suit. A neat finish needs the lining formed to the real angles, not squared off and bodged with filler. We write the lining shape and its finish into the finish schedule that comes with every quote, so the room is a planned part of the job and not left to the day.
Regulations and next steps
A new rooflight in an existing flat roof is building work, so it needs Building Control approval, and it must meet Part L. Where the roof is relaid or rafters are trimmed, that falls under the same notification. We handle the Building Control notification, and the work is covered by a 10-year workmanship guarantee.
If you are planning a rooflight over an extension or a flat-roofed kitchen, send photos of the roof and the room through the quote form. We survey the fall, the outlets and the covering before recommending a product.